One-dimensional scintillator film with benign grain boundaries for high-resolution and fast x-ray imaging

Fast and high-resolution x-ray imaging demands scintillator films with negligible afterglow, high scintillation yield, and minimized cross-talk. However, grain boundaries (GBs) are abundant in polycrystalline scintillator film, and, for current inorganic scintillators, detrimental dangling bonds at GBs inevitably extend radioluminescence lifetime and increase nonradiative recombination loss, deteriorating afterglow and scintillation yield. Here, we demonstrate that scintillators with one-dimensional (1D) crystal structure, Cs5Cu3Cl6I2 explored here, possess benign GBs without dangling bonds, yielding nearly identical afterglow and scintillation yield for single crystals and polycrystalline films. Because of its 1D crystal structure, Cs5Cu3Cl6I2 films with desired columnar morphology are easily obtained via close space sublimation, exhibit negligible afterglow (0.1% at 10 ms) and high scintillation yield (1.2 times of CsI:Tl). We have also demonstrated fast x-ray imaging with 27 line pairs mm−1 resolution and frame rate up to 33 fps, surpassing most existing scintillators. We believe that the 1D scintillators can greatly boost x-ray imaging performance.


INTRODUCTION
Scintillators are indispensable for a number of technologies, including x-ray detectors used in medical imaging and nondestructive inspection (1), phosphor screens in night-vision systems (2), electron detectors in electron microscopes (3), and electromagnetic calorimeters in high-energy physics experiments (4). Polycrystalline scintillator films are used for most of these applications, especially in x-ray flat panel detector (FPD). Research in this field is largely driven by the aim of high spatial resolution and low afterglow, for tiny-spot identification and high-speed imaging (5,6). In terms of spatial resolution, in addition to high scintillation yield, scintillators with optical wave-guiding effect have been designed. The columnar structured scintillator films enjoy low scattering and inter-pixel cross-talk and have demonstrated their intrinsic advantages in high-resolution and high-pixel fill factor for x-ray FPD (7). The afterglow effect, which causes residual shadow and image blurring, remains as a severe challenge for scintillator films. Current approaches to reduce afterglow are mostly oriented toward the doping or codoping (e.g., Sm or Bi codoping in CsI:Tl) (8,9), as well as identification of new materials (e.g., perovskites and ceramics) (10). However, these approaches either have little success or at the price of a substantially decreased scintillation yield (11,12). Hence, the development of scintillator films with combined high spatial resolution and negligible afterglow for x-ray FPD remains a long-standing challenge.
One of the major reasons that contributed to the long afterglow is the presence of defects at grain boundaries (GBs). The widely explored and commercially available scintillators (such as CsI:Tl, LYSO:Ce, and LaBr 3 :Ce) exhibit a three-dimensional (3D) crystal structure (13)(14)(15), that is, they are bound by covalent and/or coordination bonds in all three spatial dimensions. When processed into scintillation films, there are abundant GBs in this submillimeterthick film. Even with a careful posttreatment, a large amount of dangling bonds inevitably present at GBs, act as nonradiative recombination centers or afterglow traps, and deteriorate device performance (Fig. 1A). For example, for CsI:Tl polycrystalline films, the light output and the afterglow characteristics are both inferior to CsI:Tl single crystals even when the best available passivation or codopants are used, largely due to the remained abundant defects at GBs (Fig. 2E and figs. S9 to S11) (16,17).
We propose a strategy using 1D crystal structure scintillation materials to address this inherent difficulty. For this 1D scintillator, atoms are bonded via strong covalent bonds into 1D chains, and these chains are then stacked together through weak van der Waals force or inert cations that do not contribute to the valence band maximum (VBM) or conduction band minimum (CBM). As a result, carriers are mostly confined within these 1D chains, in analogy to 2D materials such as graphene or MoS 2 where carriers largely reside inside the 2D sheet (18). When processed into films, these 1D scintillators are prone to grow into columnar structures thermodynamically driven by the faster growth rate along the chains. Crucially, these films preserve chain integrity at GBs, eliminating detrimental dangling bonds and reducing nonradiative recombination losses and afterglow. We call these benign GBs, which are very beneficial for scintillator film for high-resolution and fast imaging (Fig. 1B). Recent examples, CsCu 2 I 3 and A 2 CuX 3 (A = Rb and K; X = Cl and Br), have shown potential but suffer from low luminescence efficiency [photoluminescence (PL) quantum yield (PLQY): 15.7% of CsCu 2 I 3 and 27% of Rb 2 AgBr 3 ], and the challenges in columnar film fabrication due to incongruent melting nature and the luminescence of their GBs are not studied yet (19)(20)(21). The potential of 1D materials for high-resolution, rapid scintillation imaging remains untapped.
Here, we prove the benign GBs of 1D scintillators and report the 1D Cs 5 Cu 3 Cl 6 I 2 films for high-resolution and fast x-ray imaging. Cs 5 Cu 3 Cl 6 I 2 consists of covalent bonded 1D [Cu 3 Cl 6 I 2 ] n 5n− chains that are separated by Cs + cations that have no contribution to the band edge. Moreover, Cs 5 Cu 3 Cl 6 I 2 films bear the key features that ensure its successful scintillation application. First, Cs 5 Cu 3 Cl 6 I 2 enjoys high stability against humidity and continuous radiation, and all the contained elements are cheap, nontoxic, and nonradioactive (22). The congruent melting property promises convenient fabrication via the scalable physical vapor deposition method (23). Combining with its 1D crystal structure, it is facile to achieve columnar structured film and benign GBs, thus allowing for high spatial resolution and low afterglow. The physical and scintillation properties of 1D Cs 5 Cu 3 Cl 6 I 2 films are comprehensively compared with conventional and some emerging scintillators in the radar chart ( Fig. 1C and table S1), outperforming most of scintillators.

RESULTS
We began by synthesizing Cs 5 Cu 3 Cl 6 I 2 single crystals and thick films. Previous study has reported the existence of Cs 5 Cu 3 Cl 6 I 2 phase and its PL characteristics by studying the powder state (24,25). However, the single crystals and thick films have not been  obtained, which are necessary for assessing their x-ray detection performance. Cs 5 Cu 3 Cl 6 I 2 single crystal was obtained by the vertical Bridgman method. This compound has a congruent melting nature, as evidenced by the single endothermic peak at 339°C during a heating process ( fig. S1). This property enables an easy control of the melt growth of single crystal and also the film fabrication via physical vapor deposition method. The as-polished Cs 5 Cu 3 Cl 6 I 2 crystal slabs are shown in the inset of fig. S2. The crystals are transparent, colorless, and inclusion-free. The optical bandgap was estimated as 3.74 eV ( fig. S3). Under 256-nm ultraviolet light excitation, the crystals emit bright cyan light. The x-ray diffraction (XRD) pattern and elemental mapping confirm the pure phase of Cs 5 Cu 3 Cl 6 I 2 ( fig. S4). As shown in Fig. 1B, fig. S5, and table S2, Cs 5 Cu 3 Cl 6 I 2 crystallizes into the orthorhombic crystal structure with the Cmcm space group, and the lattice parameters are a = 16.9110 Å, b = 9.1470 Å, and c = 14.0570 Å. From the view of coordination chemistry, Cu + has an electronic configuration of [Ar]3d 10 , and the outer shell takes sp 3 hybridization orbitals, while four halogen ions (Cl − and I − ) donate their lone pair electrons to coordinate with Cu + to form a tetrahedron [CuCl 2 I 2 ] unit. The role of Cs + is to balance the charge and separate the 1D zigzag [Cu 3 Cl 6 I 2 ] n 5n− chain units. According to the calculated electronic band structures and density of states, the VBM consists mainly of the localized Cu 3d orbitals and partial halogen p orbitals, rendering the valence band edge rather flat with large effective mass of holes ( fig. S6). The CBM is mainly contributed from Cu 4s orbitals with some components from halogen p orbitals, and the single I − ion bridging between [CuCl 2 I] 2 4− and [CuCl 2 I 2 ] 3− units further contributes to the localization of Cu 4s electron cloud in these units. Thus, although the crystal structure is 1D, the electronic dimensionality is quasi-zero, and carriers are strongly confined (Fig. 1D), which explains a large energy barrier for the exciton emission quenching (1087 meV; fig. S15C) and a high PL efficiency (PLQY = 91 ± 3% for our synthesized Cs 5 Cu 3 Cl 6 I 2 single crystals).
For scintillator film, the columnar wave-guiding structure is well known as one of the most effective methods to reduce the light scattering, minimize the pixel cross-talk, and enable a high spatial resolution. However, it is a great challenge to obtain columnar structured films with a high aspect ratio due to the difficulty of suppressing isotropic nucleation and crystal growth. Now, only CsI:Tl film could be reproducibly processed into columnar structure. Here, the congruent melting point of Cs 5 Cu 3 Cl 6 I 2 indicates a good phase stability in the melting state, and the 1D crystal structure makes the growth along the c direction thermodynamically favorable. We chose our previously established close-space sublimation (CSS) method to fabricate columnar structured films (26). The Cs 5 Cu 3 Cl 6 I 2 powder was heated to 600°C in 60 s and deposited onto a 6 cm-by-6 cm quartz substrate. The XRD pattern ( Fig.  2A) shows the excellent orientation along the [004] direction of the Cs 5 Cu 3 Cl 6 I 2 film, where the 1D zigzag [Cu 3 Cl 6 I 2 ] n 5n− chains are perpendicular to the substrate ( Fig. 2A, inset). The scanning electron microscopy (SEM) image shows the grain size of 2 to 5 μm in diameter ( Fig. 2A and fig. S7), which was smaller than commercial CsI:Tl film (7).
Furthermore, we use the first-principles calculations to study the atomic and electronic structure of Cs 5 Cu 3 Cl 6 I 2 crystallographic planes. The available terminations of (100) and (010) planes require no breaking of Cu─X coordination bonds, and the plane energy is expected to be lower than in the case of the planes with broken coordination bonds. The calculation results reveal that the plane energies for (100), (010), and (001) planes are 0.27, 0.34, and 0.40 J m −2 , respectively, consistent with the expectations. This indicates that the most abundant planes are the (100) and (010) faces, due to their lower formation energy and no breakage of coordination bonds, in good agreement with XRD results. For all the planes considered, no substantial plane reconstruction was observed in the calculations ( fig. S8). Following electronic structure calculation shows that no deep states are present inside the bandgap for the (100) and (010) planes ( Fig. 1E and fig. S8). Thus, the GBs will be terminated with the intrinsically benign planes, and the traps causing the afterglow and nonradiative recombination losses could be eliminated.
To support these theoretical predictions, we experimentally compare the luminescence properties of the GBs and grain interiors. The 2D topography optical microscopy image and the corresponding PL mapping of the Cs 5 Cu 3 Cl 6 I 2 films (Fig. 2, B and C) show that there is no correlation between GBs (identifiable by notable changes in topography spatial map) and the PL intensity variations. In an illustrative line crossing the GBs, the luminescence intensity of the grain interiors is 105 ± 1.28 and that of GBs is 104 ± 1.36. The average luminescence intensity difference is extremely small (1.40), which is comparable to the noise fluctuation. In comparison, for CsI:Tl scintillator film, along the illustrative line across three CsI:Tl grains with identical height, the PL intensity severely reduces from 248 ± 3.28 at the grain interiors to 233 ± 3.73 at GBs ( fig. S9), which is caused by the harmful dangling bonds at GBs.
Moreover, we measured the PLQYs of single crystal and polycrystalline films of Cs 5 Cu 3 Cl 6 I 2 and CsI:Tl. As shown in Fig. 2D, the PLQY of Cs 5 Cu 3 Cl 6 I 2 single crystal is 91 ± 3%, and the Cs 5 Cu 3 Cl 6 I 2 film exhibits similar PLQY of 93 ± 2%. By contrast, the PLQY of CsI:Tl single crystal is 98 ± 1%, while PLQY of CsI:Tl film is markedly reduced to 85 ± 1% (fig. S10), which is attributed to the light quenching by numerous surface states at the GBs (27,28). As shown in Fig. 2E, there is also no obvious difference in PL decay between Cs 5 Cu 3 Cl 6 I 2 single crystal and film, where the decay time constants can be well fitted by a single exponential function as 39.0 and 38.1 μs, respectively. The PL residual signal of Cs 5 Cu 3 Cl 6 I 2 single crystal and film under band-to-band excitation is 0.01% at 0.70 ms and 0.01% at 0.69 ms, respectively. Such a fast decrease of PL signal with the same time profile in single crystal and film indicates that there is no notable effect of additional trapping centers at GBs. In sharp contrast, for CsI:Tl, the film clearly produces a longer decay tail (time constant of 540.7 ms; fig. S11) than the single crystal. The PL residual signal of CsI:Tl single crystal is 0.01% at 1.08 ms, while in the film is 0.01% at 1.25 s, i.e., much more intense trapping effects are evident in the latter (Fig. 2E, inset).
Thermally stimulated luminescence (TSL) is a powerful technique to probe the trapping and detrapping dynamics of carriers from defects (29). Near-room temperature TSL directly reflects the deep traps, which are the culprit for afterglow effect. From the TSL glow curves as shown in Fig. 2F, the Cs 5 Cu 3 Cl 6 I 2 single crystal and film have almost the same TSL peak position and peak shape. A general order kinetic function was used to fit the glow curves and evaluate the trap parameters (30), i.e., the energetic depth of the traps, the attempt-to-escape frequency factor, and the detrapping time. The trap depth and detrapping time of the defect state in the single crystal and film are almost the same (table S3), which suggest that there is no additional defect state at GBs in the film. Notably, there is only one trap state with detrapping time in the millisecond level, which is 0.57 eV (3.10 ms) and 0.60 eV (5.26 ms) for the single crystal and film, respectively. Moreover, the glow intensity of Cs 5 Cu 3 Cl 6 I 2 single crystal in 260 to 500 K is about two orders magnitude lower than that of CsI:Tl single crystal measured at identical conditions (fig. S12), indicating a much lower deep trap density of Cs 5 Cu 3 Cl 6 I 2 .
Both theoretical simulation and experimental results revealed that, for our [001]-oriented Cs 5 Cu 3 Cl 6 I 2 films, a negligible difference was observed between films and single crystals, indicating that grain interiors and GBs are nearly indistinguishable and the GBs are intrinsically benign. We admit that some point defects (for example, I vacancies) or contamination could still exist at GBs, but defects from dangling bonds are largely eliminated, minimizing afterglow and nonradiative recombination losses. This is in sharp contrast with 3D scintillators such as CsI:Tl, where a huge difference was observed between its single crystal and film due to the abundant dangling bonds and consequently amounts of defects states at GBs. Now, we investigate the scintillation properties of Cs 5 Cu 3 Cl 6 I 2 . Cs 5 Cu 3 Cl 6 I 2 contains high-Z elements of Cs and I and has an average atomic number of 49, which guarantees high x-ray attenuation efficiency (table S4). According to the photon cross-sectional database, the attenuation coefficient of Cs 5 Cu 3 Cl 6 I 2 is comparable to that of CsI: Tl (fig. S13). The scintillation process of Cs 5 Cu 3 Cl 6 I 2 under x-ray excitation is described in Fig. 3A. High-energy electrons induced by interaction of x-ray with atoms in the Cs 5 Cu 3 Cl 6 I 2 produce an avalanche of secondary electrons that are thermalized and lastly coupled with surrounding crystal lattice to form selftrapped excitons (STEs) and emit photons. The nature of STE emission is judged from the broad emission spectrum, large Stokes shift ( fig. S14), and the soft lattice of Cs 5 Cu 3 Cl 6 I 2 . There are two emission centers (480 nm, STE1; and 590 nm, STE2) of Cs 5 Cu 3 Cl 6 I 2 under 77 K, which can be correlated with the deformation of different Cu─Cu bonds in the zigzag chains. The Huang-Rhys factors (S) of two emission centers are derived from the full width at half maximum (FWHM) of PL as 87 and 48 ( fig. S15). The large S values indicate the existence of strong electron-phonon coupling, which also support the STE emission nature. As shown in Fig. 3B, the x-ray excited radioluminescence (RL) spectrum has a single emission peak at 466 nm with an FWHM of 86 nm, which is identical to its PL spectrum ( fig. S14).
The scintillation yield of Cs 5 Cu 3 Cl 6 I 2 is calculated by comparing its 241 Am (59.5 keV) excited pulse height spectrum with that of LYSO:Ce and CsI:Tl references. The scintillators were fixed on a photomultiplier tube (PMT) with optical coupling glue. The scintillation pulses were recorded by an oscilloscope and related pulse height spectra are shown in Fig. 3C. The scintillation yield is proportional to the channel number of the full energy peak centroid and was further corrected by considering the emission-weighted quantum efficiency of PMT for these three different scintillators. The scintillation yield of Cs 5 Cu 3 Cl 6 I 2 is 2.8 times of benchmarking LYSO:Ce (24,000 photons/MeV) and 1.2 times of CsI:Tl sample (54,000 photons/MeV). Thus, the scintillation yield of Cs 5 Cu 3 Cl 6 I 2 is estimated within 64,800 to 67,200 photons/MeV. High scintillation yield allows scintillator detector to maintain high signal-tonoise ratios (SNRs) under low-dose irradiation. The detection limit of Cs 5 Cu 3 Cl 6 I 2 achieves 11 nanogray equivalent air (nGy air ) s −1 (Fig. 3D), which is one of the lowest among all kinds of scintillator and comparable to semiconductor x-ray detectors (CsPbBr 3 quantum dots, 13 nGy air s −1 ; Cs 0.15 FA 0.85 PbI 3 semiconductor detector, 13.8 nGy air s −1 ; and CsPbBrI 2 , 1 nGy air s −1 ) (10,31,32), and is only 1/500 of the dose rate used in traditional x-ray medical diagnostics (5.5 μGy air s −1 ) (33).
We recorded the scintillation afterglow of Cs 5 Cu 3 Cl 6 I 2 and CsI:Tl film under x-ray excitation (Fig. 3E). After x-ray cutoff, the afterglow signal of Cs 5 Cu 3 Cl 6 I 2 attenuates by nearly three orders of magnitude within 10 ms (0.1% at 10 ms), which is lower than CsI:Tl (0.4% at 10 ms) and comparable to Gd 2 O 2 S:Tb (table S1). In longer time scale (100 s), it attenuates nearly four orders of magnitude and is over one order of magnitude lower than that of CsI:Tl (fig. S17). As shown in the fig. S12, the CsI:Tl has high intensity glow peaks in 260 to 500 K, which refer to the deep trap states and result in unacceptable afterglow within seconds to minutes. We also measured the RL decay profile of Cs 5 Cu 3 Cl 6 I 2 film under x-ray excitation (Fig. 3E,  inset). The RL decay can be well fitted by a single exponential function with a time constant of 43.5 μs, similar to the PL decay lifetime of 37.4 μs (fig. S16). The similar lifetime and the same RL and PL spectrum indicate that no additional recombination pathway contributes to the scintillation emission mechanism.
We also study the stability of Cs 5 Cu 3 Cl 6 I 2 crystal toward moisture and continuous irradiations (Fig. 3F). There is no weight change after 12 hours of exposure under a relative humidity 60%. In sharp contrast, the weight of NaI:Tl rapidly increases by 0.18% under the same condition. For radiation stability, the single crystal was exposed under x-rays in ambient atmosphere (25°C, 70% humidity) without any encapsulation, and its light output was recorded. As shown in the red dots of Fig. 3F, 8.5 hours of xray radiation with an accumulated total dose of 612 Gy air did not deteriorate the light output of Cs 5 Cu 3 Cl 6 I 2 .
Last, we studied the x-ray imaging performance of Cs 5 Cu 3 Cl 6 I 2 film. We note that, through our CSS process, the Cs 5 Cu 3 Cl 6 I 2 film could be easily and reproducibly deposited onto various substrates, including quartz (Fig. 4A), flexible polyimide ( fig. S18), or thin-film transistor (TFT) panels due to its congruent melting property. According to the slanted-edge method, a high spatial resolution of 27.1 line pairs per millimeter (lp mm −1 ) at a modulation transfer function (MTF) of 0.2 is obtained for the Cs 5 Cu 3 Cl 6 I 2 film (Fig. 4B). This value far exceeds commercial CsI:Tl (3 to 5 lp mm −1 ) and GOS:Tb (1.7 lp mm −1 ) and also exceed that of the recently reported record spatial resolution of perovskite (16.8 lp mm −1 ) and organic scintillators (18 lp mm −1 ) (10,(34)(35)(36)(37)(38). The high spatial resolution can be attributed to the smaller columnar crystals, which reduce optical cross-talk (39). Furthermore, we took photos of a standard x-ray resolution pattern plate (QUART Type 74, 5.0 to 20 lp mm −1 ) and gold wires with 15 μm diameter (Fig. 4C and fig. S19). The results demonstrate that the line pair notches at 20 lp mm −1 and the gold wires are clearly distinguishable, echoing the results by the slanted-edge method.
For imaging application, the detective quantum efficiency (DQE) is an important metric related to the image quality, which refers to the efficiency of an x-ray detector in converting x-ray photons into imaging signal. As shown in Fig. 4D, the DQE(0) of Cs 5 Cu 3 Cl 6 I 2 film is about 80%, which is larger than that of commercial CsI:Tl-based complementary metal-oxide semiconductor (CMOS) FPD (60 to 75%) and GOS:Tb-based FPD (35%) (38,40). In addition, the DQE of the Cs 5 Cu 3 Cl 6 I 2 film decreases slowly with spatial resolution and remains above 20% even at 23 lp mm −1 . The high DQE can primarily be attributed to the exceptional spatial resolution of the Cs 5 Cu 3 Cl 6 I 2 . High DQE ensures that the Cs 5 Cu 3 Cl 6 I 2 film maintains a good contrast for small objects imaging and enables its competitive application in x-ray FPD. We thus demonstrated the high-quality x-ray images of a print circuit board and a spring (fig. S20). The outline of the invisible connection cable and spring were clearly presented. Our Cs 5 Cu 3 Cl 6 I 2 films have minimized recombination loss at GBs and high scintillation yield, enabling large SNR and hence large DQE and sensitive detection; they also have columnar structure with a large refractive index (1.77 at 410 nm; fig. S21), resulting in a negligible cross-talk and hence a high spatial resolution. It is also worth mentioning that the Cs 5 Cu 3-Cl 6 I 2 film also exhibits high stability. After being exposed to air for 3 months, there was no noticeable change observed in its PLQY ( fig.  S22A). Furthermore, the light output and imaging ability remain excellent even after continuous irradiation ( fig. S22, B and C).
As stated before, the Cs 5 Cu 3 Cl 6 I 2 scintillator enjoys a low afterglow. We used a camera to acquire residual images of a screw after 10 s of x-ray lasting exposition. As shown in Fig. 4E, the Cs 5 Cu 3 Cl 6 I 2 film shows virtually no residual image in any of the frames, whereas the commercial CsI:Tl film shows a clearly residual image even after 3 s.
Last, we showcase our Cs 5 Cu 3 Cl 6 I 2 film for high-resolution and fast x-ray imaging, a long-sought goal for x-ray FPD. As shown in Fig. 4F, the chips (fig. S23A) were placed on a conveyor belt and xray was irradiated continuously. The camera recorded the image every 40 ms. As the chip moved, the chip's internal structure could be seen very clearly and without any residual shadow ( Fig.  4G and movie S1). Moreover, we also simulated the applications of Cs 5 Cu 3 Cl 6 I 2 film in angiography, which is a medical imaging technique used to visualize the blood vessels. During angiography, a radio-opaque contrast agent needs to be injected into the body, such as iodinated contrast media, flowing through the heart to the whole body (Fig. 4H). Here, we simulate the process by injecting the iodinated contrast media into a hose ( fig. S23, B to D). The flow position of the contrast media in the hose can be clearly observed with a frame interval of 30 ms (Fig. 4I and movie S2). This speed outperforms traditional angiographic imaging in clinical settings and substantially reduces the stroboscopic effects (41). The detection of blood flow rate by our Cs 5 Cu 3 Cl 6 I 2 scintillator is remarkable as this is indispensable for the diagnosis and monitoring of cardiovascular disease.

DISCUSSION
In summary, we successfully developed a class of x-ray imagers based on 1D scintillator film and demonstrated its unprecedented capability for high-resolution and fast x-ray imaging. Compared to conventional 3D scintillators that suffer from defects at GBs, 1D scintillators are prone to grow into columnar films with inert planes exposed and, thereafter, enjoy benign GBs and enable minimized recombination loss and suppressed afterglow. Our rationally selected 1D scintillator Cs 5 Cu 3 Cl 6 I 2 exhibits the long-sought combination of high scintillation yield (1.2 times of CsI:Tl), low afterglow (0.1% at 10 ms), columnar growth capability, low toxicity, and good stability. As a result, our Cs 5 Cu 3 Cl 6 I 2 x-ray imager achieved a high spatial resolution of 27.1 lp mm −1 and a DQE(0) of 80%, exceeding nearly all reported scintillators. Moreover, the concept of 1D scintillator can be further exploited for other high-speed and high-spatial resolution x-ray imaging applications.

Cs 5 Cu 3 Cl 6 I 2 single crystal
High-purity CsCl, CuCl, and CuI were used as received. The raw materials were mixed and loaded in a 7-mm-diameter quartz ampoule in an Ar glovebox according to the stoichiometric of 5:1:2 for CsCl:CuCl:CuI. The ampoule was sealed under a vacuum of 10 Pa and placed into the vertical Bridgman growth furnace. The growth temperature was about 330°C. The temperature gradient of about 20°C cm −1 and the growth rate of 0.5 mm h −1 were used. After the crystal growth was completed, the furnace temperature was cooled to room temperature in 24 hours.

Cs 5 Cu 3 Cl 6 I 2 film
The Cs 5 Cu 3 Cl 6 I 2 film was deposited on a quartz glass or polyimide substrate by a CSS system (Qihui Vacuum Technology Co. Ltd. Shenyang, China). The graphite boat source was filled with 0.8 g of Cs 5 Cu 3 Cl 6 I 2 powder, which was obtained by grinding the single crystals. The source and the substrate were placed within a quartz tube, and the whole system was kept under a vacuum below 2 × 10 −3 Pa. The source was heated to 600°C within 60 s via infrared radiation. The sublimated Cs 5 Cu 3 Cl 6 I 2 species condensed on the substrate to form the Cs 5 Cu 3 Cl 6 I 2 film with a thickness of 30 μm.

Theoretical calculation
The density functional theory calculations were performed by the Vienna Ab initio Simulation Package (42). The Perdew-Burke-Ernzerhof generalized gradient approximation was used for the exchange-correlation functional (43). Projector-augmented wave potentials were used to describe the interactions between ion cores and valence electrons (44). Plane waves with cutoff energy of 22.5 Ry were taken as the basis set. For the electronic structure calculation of bulk materials, we used experimentally determined crystal structure. The first Brillouin zone was sampled by a Γ-centered 3 × 4 × 3 k-point grid. To calculate the plane energy, plane structures based on 2 × 1 × 1, 1 × 2 × 1, and 1 × 1 × 2 supercells were constructed for (100), (010), and (001) planes, respectively. A vacuum layer of at least 10 Å was used. The atoms were relaxed until the residual forces on all atoms were smaller than 0.01 eV. Then, the plane energy was calculated on the basis of a Γ-centered k-point grid of 4 × 2 × 1.

Material characterization
Single-crystal x-ray diffraction was collected with a graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) at room temperature on a Bruker-Lynxeye diffractometer. The data reduction and multiscan absorption correction were all performed using the CrysAlis-Pro software supplied by the manufacturer. Ab initio structure solutions were performed by the Direct Methods algorithm using ShelXS program. All structures were refined by full-matrix least squares using ShelXL program. The morphology of the cross section and EDS mapping was studied by SEM (GeminiSEM 300, Carl Zeiss) with a 10-kV accelerating voltage. The differential scanning calorimetry was conducted by using a Netzsch STA449F3 instrument. About 50 mg of the single-crystal sample was heated to 400°C in Al 2 O 3 crucible at a rate of 5°C/min under the condition of ultrahigh purity argon atmosphere.
The PLE and emission (PL) spectra, emission and excitation contour mapping, and RL spectrum and x-ray-induced afterglow spectra at room temperature were measured with a Horiba FluoroMax+ spectrofluorometer. A Xe steady-stay lamp was used as the excitation source for PL measurements. For RL measurements, an x-ray tube (50 kV, 500 mA, KYW900A) with W anode was used as the excitation source. An integrating sphere was equipped to collect the emissions, which was fed through an optical fiber to the receiving end of the spectrofluorometer with dual scanning monochromators. The PL decay spectrum was recorded by a Quan-taMaster 8000 (HORIBA Scientific, Canada) with an excitation wavelength at 290 nm. The PLQY was determined by an absolute quantum yield measurement system attached with an integrating sphere and excited by 271-nm wavelength (Hamamatsu Quantaurus-QY). The PL mapping was measured by a fluorescence microscope (BX53, Olympus, Japan). TSL was measured by ROSB TL 3DS thermoluminescence spectrometer. The CsI:Tl and Cs 5 Cu 3 Cl 6 I 2 single crystal has the same size of 4 mm by 4 mm by 2 mm. The samples were irradiated by x-ray (50 kV, 500 mA) for 10 min at 80 K and 30 min of waiting and then heated at a heating rate of 0.1 K s −1 . The curves were fitted by following equation where E is the trap depth, s is the frequency factor, n 0 is the initial concentration of trapped charges, l is the kinetic order, k B is the Boltzmann constant, and β is the heating rate. The PLE and emission (PL) spectra at 77 K and the temperature dependence spectra were measured by Xe lamp excitation via Edinburgh Instruments FLS 980 spectrometer with a temperature controller for liquid nitrogen (77 to 500 K) cryostat. The activation energy (E a ) was estimated by fitting the integrated PL intensity under different temperatures with the following equation where I 0 is the integral intensity at 0 K, Γ ν is the radiative decay rate, Γ 0 is the attempt rate of thermal quenching, and k B is the Boltzmann constant. The Huang-Rhys factor (S) was obtained by nonlinear fitting the temperature dependent FWHM with the equation (45) FWHM where ℏω phonons is the phonon frequency. The refractive index was measured by a spectroscopic ellipsometry (SE-VM-L, Wuhan Eoptics Technology Co. Ltd.) with measurement wavelength of 380 to 1000 nm. The light spot diameter was 200 μm. A Cs 5 Cu 3 Cl 6 I 2 single crystal with 8-mm diameter and 2-mm thickness was used in the measurement. The crystal was polished and guaranteed that the top and bottom surfaces were parallel to each other.

Scintillation characterization
The scintillation yield was calculated by comparison of the 241 Am pulse height spectra. The Cs 5 Cu 3 Cl 6 I 2 crystal was a cylinder with a diameter of 7 mm and a thickness of 3 mm. The CsI:Tl crystal (purchased from Scionix) was a cube with a size of 3 mm by 3 mm by 5 mm, and the size of LYSO crystal was 3 mm by 3 mm by 10 mm. The density of Cs 5 Cu 3 Cl 6 I 2 , CsI:Tl, and LYSO was 3.8, 4.5, and 7.1 g cm −3 , respectively. The crystal was fixed on a PMT (R2059, Hamamatsu) by optical coupling glue. A high voltage power supply (556, Ortec) was used to supply voltage to the PMT. An oscilloscope (MSO54B 5-BW-2000, Tektronix) was used to read the output signal of the PMT. The 241 Am with radioactivity of 2.85 × 10 8 Bq irradiated the crystal through a Be window from a distance of 1 cm. The PMT and 241 Am were placed in shielded boxes and completely protected from light. A total of 5000 pulses were collected by the oscilloscope and processed as pulse height spectra and fitted by Gauss function. The relative scintillation yield of Cs 5 Cu 3 Cl 6 I 2 was obtained by following equation where LY R was the relative scintillation yield, C j (j = Cs 5 Cu 3 Cl 6 I 2 / CsI:Tl/LYSO) was the channel number, and ϕ j was the PMT spectral quantum efficiency ( fig. S24A) correction constant. The PMT spectral quantum efficiency correction constant was calculated by where I j (λ) was the RL intensity of scintillator ( fig. S24B) and S(λ) was the wavelength-dependent detection efficiency of PMT. For the detection limit measurement, the Cs 5 Cu 3 Cl 6 I 2 was attached to a silicon photomultiplier (JSP-TN3050-SMT), and the data were collected by an oscilloscope (DSO-S 054A, Keysight). The x-ray dose rates were adjusted by changing the current of the x-ray tube (10 to 80 mA, 50 kV; Leo, Varex Imaging) and increasing the Cu filter (0 to 2 mm). The dose rates were calibrated with an ion chamber (10X6-180, Radcal) and a solid-state sensor (DDX6-WL). We measured the scintillator response under dose rates of 9.05, 11.31, 14.56, 18.11, 23.21, 28.11, and 36.22 nGy air s −1 . The detection limit was derived when the SNR of the response equaled to 3. As shown in fig. S25, the "signal" was calculated by subtracting the average value of the dark voltage from the photovoltage. The "noise" was the SD of the photovoltage.
The scintillation decay was obtained by using x-ray tube (70 kV, 10 ns of pulse width) with W anode as excitation source, an optical fiber to transmit the light to PMT (2 ns of rise time), and a digital phosphor oscilloscope (1 GHz of bandwidth) to get the curve.
The afterglow was obtained by using x-ray tube (10 kV, 50 mA, Seifert Gmbh) for excitation and collecting the spectrally unresolved emission by 5000 M model of Horiba Jobin Yvon fluorometer running in the multichannel scaling mode and equipped with TBX-04 photon counting detector (IBH-Scotland).

X-ray imaging
We assembled an imaging system in a black lead box. The x-ray source used in the system was M237 (50 kV, Newton Scientific) with Au target. The Cs 5 Cu 3 Cl 6 I 2 film was placed on a reflector (CCM1-G01, Thorlabs), and the scintillation light was deflected to a CMOS camera (C13440, Hamamatsu) with pixel size of 6.5 μm by 6.5 μm. For the demonstration of chip pipelining inspection, a chip was fixed to a scanning stage (PSA200-11-X, Zolix) at a step rate of 4.5 mm s −1 . The camera was set at automatic acquisition mode. For the angiography simulation, a processing approach typical of digital subtraction angiography was applied. A "mask image" was captured before the injection of the contrast agent. This mask image was then subtracted from the subsequent image taken after contrast injection, yielding a clear angiographic image. The pixel values of scintillation photon noise and the camera's electronic noise were calculated as 6.4 ( fig. S26).
DQE was calculated with MIQuaELa software and measured following the guidance of the international standard IEC62372. The noise power spectrum (NPS; fig. S27) and MTF were measured with RQA3 beam, which was produced form a tube (Leo, Varex Imaging) voltage of 50 kV. The detector responses were measured under several different doses to obtain signal transfer property. The absorption dose of 2.5 μGy air was used to measure the NPS of the detector.

Supplementary Materials
This PDF file includes: Supplementary Text Figs. S1 and S27 Tables S1 to S4 Legends for movies S1 and S2 References Other Supplementary Material for this manuscript includes the following: Movies S1 and S2